(Sources and citations provided by Ali Kuoppala of www.beardresource.com)
If you have a beard you probably enjoy using beard oil and various other beard products like balms and beard soap.
But did you know that many of the essential oils used as ingredients in beard products can suppress your growth and harm the beard? (some of these can be found on tumblr of the Beard Resource).
How this happens is simple; most of the oils are highly polyunsaturated and volatile. They are prone to lipid-peroxidation, which makes them go rancid when exposed to oxygen, heat, and light + some of the compounds like eucalyptol, 4-terpineol, d-limonene, and alpha-terpineol have direct androgen suppressing effect.
As you might know, facial hair growth is driven by androgenic hormones, and anything that blocks the effects of those hormones, will directly go against your beard growing abilities.
Studies suggest that using eucalyptus oil for beard growth is a bad idea, as it has really high amounts of DHT blocking eucalyptol.
Tea tree oil is also bad for beard growth, primarily because it contains so powerful antiandrogenic compounds that it has been linked to increased gynecomastia in young boys.
And then there's Amla oil, which is popular in India and unfortunately really bad for your beard growth, as it is one of the most potent known 5-alpha reductase enzyme blockers, which results in lowered DHT and weaker beard growth.
Sad news are that many of these oils are used as beard oil ingredients, and some companies even claim that they would promote facial hair growth...
This is absolutely insane, when the evidence clearly suggests that these essential oils do the opposite...
...They suppress your facial hair growth by reducing androgenic hormones.